在我的 Ubuntu 14.04 上，我遵循http://mirror.its.dal.ca/cran/bin/linux/ubuntu/README将以下内容添加到/etc/apt/sources.list
deb http://stat.ethz.ch/CRAN/bin/linux/ubuntu trusty/
当我运行时sudo apt-get update，出现此错误
W: GPG error: http://stat.ethz.ch trusty/ Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 51716619E084DAB9
整个输出在这篇文章的末尾。我想知道这个问题是什么意思以及如何解决它？谢谢！
$ sudo apt-get update
Ign http://us.archive.ubuntu.com trusty InRelease
Ign http://us.archive.ubuntu.com trusty-updates InRelease
Ign http://dl.google.com stable InRelease
Ign http://us.archive.ubuntu.com trusty-backports InRelease
Ign http://security.ubuntu.com trusty-security InRelease
Hit http://us.archive.ubuntu.com trusty Release.gpg
Hit http://security.ubuntu.com trusty-security Release.gpg
Hit http://us.archive.ubuntu.com trusty-updates Release.gpg
Ign http://dl.google.com stable InRelease
Hit http://us.archive.ubuntu.com trusty-backports Release.gpg
Hit http://security.ubuntu.com trusty-security Release
Hit http://dl.google.com stable Release.gpg
Hit http://us.archive.ubuntu.com trusty Release
Hit http://us.archive.ubuntu.com trusty-updates Release
Hit http://security.ubuntu.com trusty-security/main Sources
Ign http://extras.ubuntu.com trusty InRelease
Hit http://dl.google.com stable Release.gpg
Hit http://us.archive.ubuntu.com trusty-backports Release
Ign http://archive.canonical.com trusty InRelease
Hit http://security.ubuntu.com trusty-security/restricted Sources
Hit http://us.archive.ubuntu.com trusty/main Sources
Hit http://dl.google.com stable Release
Hit http://us.archive.ubuntu.com trusty/restricted Sources
Hit http://security.ubuntu.com trusty-security/universe Sources
Hit http://us.archive.ubuntu.com trusty/universe Sources
Hit http://security.ubuntu.com trusty-security/multiverse Sources
Hit http://security.ubuntu.com trusty-security/main amd64 Packages
Hit http://dl.google.com stable Release
Hit http://us.archive.ubuntu.com trusty/multiverse Sources
Hit http://extras.ubuntu.com trusty Release.gpg
Hit http://security.ubuntu.com trusty-security/restricted amd64 Packages
Hit http://us.archive.ubuntu.com trusty/main amd64 Packages
Ign http://archive.canonical.com trusty InRelease
Hit http://security.ubuntu.com trusty-security/universe amd64 Packages
Hit http://us.archive.ubuntu.com trusty/restricted amd64 Packages
Hit http://dl.google.com stable/main amd64 Packages
Hit http://security.ubuntu.com trusty-security/multiverse amd64 Packages
Hit http://us.archive.ubuntu.com trusty/universe amd64 Packages
Ign http://stat.ethz.ch trusty/ InRelease
Hit http://security.ubuntu.com trusty-security/main i386 Packages
Hit http://us.archive.ubuntu.com trusty/multiverse amd64 Packages
Hit http://dl.google.com stable/main i386 Packages
Hit http://security.ubuntu.com trusty-security/restricted i386 Packages
Hit http://us.archive.ubuntu.com trusty/main i386 Packages
Hit http://us.archive.ubuntu.com trusty/restricted i386 Packages
Hit http://security.ubuntu.com trusty-security/universe i386 Packages
Hit http://extras.ubuntu.com trusty Release
Hit http://archive.canonical.com trusty Release.gpg
Hit http://security.ubuntu.com trusty-security/multiverse i386 Packages
Hit http://security.ubuntu.com trusty-security/main Translation-en
Hit http://security.ubuntu.com trusty-security/multiverse Translation-en
Hit http://us.archive.ubuntu.com trusty/universe i386 Packages
Hit http://security.ubuntu.com trusty-security/restricted Translation-en
Get:1 http://stat.ethz.ch trusty/ Release.gpg [490 B]
Hit http://dl.google.com stable/main amd64 Packages
Hit http://us.archive.ubuntu.com trusty/multiverse i386 Packages
Hit http://archive.canonical.com trusty Release.gpg
Hit http://extras.ubuntu.com trusty/main Sources
Hit http://dl.google.com stable/main i386 Packages
Hit http://us.archive.ubuntu.com trusty/main Translation-en
Hit http://security.ubuntu.com trusty-security/universe Translation-en
Hit http://us.archive.ubuntu.com trusty/multiverse Translation-en
Hit http://stat.ethz.ch trusty/ Release
Ign http://stat.ethz.ch trusty/ Release
Hit http://extras.ubuntu.com trusty/main amd64 Packages
Hit http://us.archive.ubuntu.com trusty/restricted Translation-en
Hit http://archive.canonical.com trusty Release
Hit http://us.archive.ubuntu.com trusty/universe Translation-en
Hit http://us.archive.ubuntu.com trusty-updates/main Sources
Hit http://extras.ubuntu.com trusty/main i386 Packages
Hit http://us.archive.ubuntu.com trusty-updates/restricted Sources
Hit http://archive.canonical.com trusty Release
Hit http://us.archive.ubuntu.com trusty-updates/universe Sources
Ign http://stat.ethz.ch trusty/ Packages/DiffIndex
Hit http://us.archive.ubuntu.com trusty-updates/multiverse Sources
Hit http://us.archive.ubuntu.com trusty-updates/main amd64 Packages
Hit http://us.archive.ubuntu.com trusty-updates/restricted amd64 Packages
Hit http://us.archive.ubuntu.com trusty-updates/universe amd64 Packages
Hit http://archive.canonical.com trusty/partner Sources
Hit http://us.archive.ubuntu.com trusty-updates/multiverse amd64 Packages
Hit http://us.archive.ubuntu.com trusty-updates/main i386 Packages
Ign https://private-ppa.launchpad.net precise InRelease
Hit http://archive.canonical.com trusty/partner amd64 Packages
Hit http://us.archive.ubuntu.com trusty-updates/restricted i386 Packages
Hit http://us.archive.ubuntu.com trusty-updates/universe i386 Packages
Hit http://us.archive.ubuntu.com trusty-updates/multiverse i386 Packages
Hit http://us.archive.ubuntu.com trusty-updates/main Translation-en
Hit https://private-ppa.launchpad.net precise Release.gpg
Hit http://us.archive.ubuntu.com trusty-updates/multiverse Translation-en
Hit http://us.archive.ubuntu.com trusty-updates/restricted Translation-en
Hit http://archive.canonical.com trusty/partner i386 Packages
Hit http://us.archive.ubuntu.com trusty-updates/universe Translation-en
Hit http://us.archive.ubuntu.com trusty-backports/main Sources
Hit http://us.archive.ubuntu.com trusty-backports/restricted Sources
Hit http://us.archive.ubuntu.com trusty-backports/universe Sources
Hit http://us.archive.ubuntu.com trusty-backports/multiverse Sources
Hit http://stat.ethz.ch trusty/ Packages
Hit https://private-ppa.launchpad.net precise Release
Hit http://us.archive.ubuntu.com trusty-backports/main amd64 Packages
Hit http://us.archive.ubuntu.com trusty-backports/restricted amd64 Packages
Hit http://us.archive.ubuntu.com trusty-backports/universe amd64 Packages
Hit http://us.archive.ubuntu.com trusty-backports/multiverse amd64 Packages
Hit http://us.archive.ubuntu.com trusty-backports/main i386 Packages
Hit http://us.archive.ubuntu.com trusty-backports/restricted i386 Packages
Ign http://dl.google.com stable/main Translation-en_US
Hit http://archive.canonical.com trusty/partner Sources
Hit https://private-ppa.launchpad.net precise/main amd64 Packages
Ign http://dl.google.com stable/main Translation-en
Hit http://us.archive.ubuntu.com trusty-backports/universe i386 Packages
Ign http://dl.google.com stable/main Translation-en_US
Hit http://us.archive.ubuntu.com trusty-backports/multiverse i386 Packages
Hit http://us.archive.ubuntu.com trusty-backports/main Translation-en
Hit http://archive.canonical.com trusty/partner amd64 Packages
Hit https://private-ppa.launchpad.net precise/main i386 Packages
Hit http://us.archive.ubuntu.com trusty-backports/multiverse Translation-en
Ign http://dl.google.com stable/main Translation-en
Hit http://us.archive.ubuntu.com trusty-backports/restricted Translation-en
Hit http://us.archive.ubuntu.com trusty-backports/universe Translation-en
Hit http://archive.canonical.com trusty/partner i386 Packages
Ign http://extras.ubuntu.com trusty/main Translation-en_US
Ign http://us.archive.ubuntu.com trusty/main Translation-en_US
Ign http://us.archive.ubuntu.com trusty/multiverse Translation-en_US
Ign http://us.archive.ubuntu.com trusty/restricted Translation-en_US
Ign http://extras.ubuntu.com trusty/main Translation-en
Ign http://us.archive.ubuntu.com trusty/universe Translation-en_US
Ign http://archive.canonical.com trusty/partner Translation-en
Ign http://stat.ethz.ch trusty/ Translation-en_US
Ign http://archive.canonical.com trusty/partner Translation-en
Ign http://stat.ethz.ch trusty/ Translation-en
Ign https://private-ppa.launchpad.net precise/main Translation-en_US
Ign https://private-ppa.launchpad.net precise/main Translation-en
Fetched 490 B in 6s (78 B/s)
Reading package lists... Done
W: GPG error: http://stat.ethz.ch trusty/ Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 51716619E084DAB9
答案1
查看自述文件：
安全 APT
CRAN 上的 Ubuntu 档案使用“Michael Rutter <[电子邮件保护]>”，密钥 ID 为 E084DAB9。要使用一个命令将密钥添加到您的系统，请使用（感谢 Brett Presnell 提供的提示）：
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys E084DAB9可以使用另一种方法检索密钥
gpg --keyserver keyserver.ubuntu.com --recv-key E084DAB9然后将其
apt-key喂给gpg -a --export E084DAB9 | sudo apt-key add -